A typical computer system includes a computer that is interfaced with a number of peripheral devices. The peripheral devices can be accessed by an operating system executing on the computer or by application programs executing on the computer via an appropriate interface. Such access to the devices is controlled by software programs known as device drivers. Many device drivers, such as those provided by the Microsoft MS-DOS operating system, are "real mode device drivers" which operate in a processor mode known as real mode. Real mode is the only mode of operation provided by the Intel 8086 microprocessor, on which the Microsoft MS-DOS operating system was originally designed to execute.
More recent microprocessors such as the Intel 80826, 80386 and 80486, sold by Intel Corporation of Cupertino, Calif., are upwardly compatible with the 8086 and thus are designed to support real mode but they may also operate in an enhanced mode, known as "protected mode". Protected mode provides benefits not provided by real mode. For instance, protected mode provides an enlarged virtual address space than provided by real mode. Protected mode also provides hardware support for multitasking and data security, which real mode does not. (The protection mechanism which provides data security in the 80386 and 80486 is described in pp. 101-133 of "Microsoft's 80386/80486 Programming Guide," published by Microsoft press, 1991.)